Recent sediment accumulation in the Norwegian Channel, North Sea

HENK DEHAAS, ELLEN OKKELS& TJEERD C.E. VAN WEERING

de Haas, H.,Okkels, E. & van Weering, T.C.E. 1996: Recent sediment accumulation in the Norwegi an Channel, North Sea. Nor. geol. un ders. Bull. 430,57-65 .

In the Norwegian Channel 35 box cores were taken for sedimento logical analysis, and 'lOPO (J.- and '37CSy-spect ros­ copy to determine sedimentation rates. Grain sizes were determined and X-ray photog raphs of th e cores were made. In addit ion, penetrat ing echo sounding lines covering a length of more th an 5500 km were studied. Sediments enter the Norwegian Channel from th e south and th e west as suspended load. The dry bulk densit ies of these generally fine-grained sediments (silty clays - silts) range from 0.35 to 1.79 qxcrn', Most of the cores show a surface mixed layer of less than 2 cm. The sedimentation rates measured in the Norweg ian Channel range from 30 to 280 mmx100 yr', Highest sedimentation ratesare foun d in the north ern part of the research area.The total recent dry bulk sediment accumulation in t he Norweg ian Channel is calculated to be 28x1O· tons'yr' . Penetrating echoso­ under data reveal th at sedimentat ion occurs in t he deeper parts and in small protected basins along the flanks of th e Norwegian Channel. The contrast between th e present-day relat ively high sedimentat ion rates and t he thin Holocene sedimentary unit is explained by a change in th e depo sit ional system somet ime during th e Holocene.

de Haas, H., Okkels, E.& van Weering, T.CE., Netherlands Institu te for 5ea Research, P.O.Bax 59, 7790 AB Den Burg, The Netherlands.

Introduction ment load of the North Sea . The sediments in the North Sea are transported by an anticlockwise residual circulati­ The Norwegian Channel is one of the largest and deepest on (Otto et al. 1990). Recent sediment accumulation recent sedimentary basins of the North Sea and forms a occurs mainly along the eastern margin of the North Sea major sink for fine-g rained material in th e North Sea (t he Wadden Sea, the German Bight, the Skagerrak, the (Eisma 1981, van Weering 1981, Eisma & Kalf 1987, van Kattegat and the Norwegian Channel) (McCave 1973, Weering et al. 1987, 1993). Sediments enter t he North Sea Eisma 1981, Eisma &Kalf 1987). (Fig. 1) from the Atlantic Ocean in the north, through the In recent years, application of 210 Pb geoch ronology has English Channel, from the Baltic Sea and from river input. been used to calculate sedimentat ion and/or accumulati­ Furthermore, sea floor erosion, coastal erosion, prima ry on rates of recent sediments in the depos itional sinks of production and atmospheric input contribut e to the sedi- the North Sea (van Weering et al. 1987, 1993, Zuo et al. 1989, Jorgensen et al. 1990, Denneqard et al. 1992a). Other met hods have also been used, such as pollen studi ­ o 10 20 es (Zagwij n & Veenstra 1966, van Weering 1982, Henningm oen &Heeq 1985, Long et al. 1988), mass bud­ get calculations of suspended sediments (E isma 1981), 14C (Long et al. 1988, Moodley &van Weering 1993), dinofla­ gellates (Long et al. 1986), stable isotopes (Erlenkeuser 6 0 1985) and foraminifera (van Weering 1982, van Weering & 6 0 Ovale 1983, Ovale & van Weering 1985). In th is paper we report on sedimentation and sedi­ ment accumu lation rates in the Norwegian Channel based on data collected dur ing a cruise with the R.V. 'Aurelia' in 1988 and two cruises with the R.V. ' Pelagia' in 1991 and 1993 (ENAM93) to the Norwegian Channel, and on data from th e literature.This st udy has aimed at deter­ mining t he total amount of sediment being deposited in th e Norwegian Channel, and to explain the major sedi­ mentary processes.

5 0 Methods 5 0 o 10 20 A total of 35 box cores were collected in the Norwegian Channel with the RV. 'Aurelia' (1988) and with the R.V. ' Fig. 1. The North Sea and the study area (outlined). 58 Henk de Haa s, El/en Okkels &Tjeerd c.E. van Weerin g GU-BULL 430. 1996

o 2 4 6 ographs were used to check the cores for bioturbation and other possible disturba nces. Sedimentation rates were determined only fro m cores not disturbed by bio­ turbation or by sampling and/o r storage . Twenty-fiv e cores were considered suitable for sedimentation rate 62 determination (Fig. 2). Grain-size analyses were perfor­ med at the University of Utrecht using a Malvern Particle 3 • Sizer model 26000 capable of measuring grain sizes w it­ 1P6 14 13 hin a range of 0.5-188 mm. Ory bulk densities were deter­ o DD 11 10 7 mined by taking subsamples of 5 cm' of wet sedim ent which were dried at 60-700( for 72 hour s and aft erwards weighed on a Mettl er P1200N balance. Sediment ation rates and dry bulk densities were corrected fo r compa ct i­ on during subsampling and storage of the samples. This was done by measuring the compaction of the sediments 60 in the liners and assuming a linear compaction over th e whole sediment column. Accumulation rates were calcu­ lated using sedimentation rates and dry bulk densit ies. 3.5 kHz penetrating echosounder lines recorded in 1973, North Sea 1974, 1975, 1976 and 1993, and data from the literature were used to determine where in the Norweg ian Channel recent sedimentation and erosion/ non-deposition occurs.

Geolog ical setting, hydrography and 58+------. r:?'station number sediment transport t:. NG88 5 • Sk91 o The present morphology of the Norweg ian Channel is the o ENAM 938~ed i me n tat io n rate (cm·100 yr 1) product of glacial erosion followed by sedimentation during the Pleistocene and Holocene (van Weering 1975, Otto et al. 1990, Pedersen et al. 1991, Holteda hl 1993, Fig. 2. Locations and sedime ntation rates for the station s where sedimenta­ Pederstad et al. 1993). At - 59°30'N the Norwegian tion rates have been determ ined. Channel shallows to 280 m at a saddle po int. Further sout h, the Norwegian Channel deepens to more than 700 m in the Skagerrak (Otto et al. 1990). Pelagia' (1991 and 1993). Immediately after retrieval of An overv iew of the dominant circulation pattern in the the cores the overlying bottom water was siphoned off northern North Sea is given by Otto et al. (1990). Atlant ic and subsamples were made by inserting wet PVC liners water enters the North Sea mainly along th e western slo­ (0 : 9 cm) into the box cores. The liners were closed with pe of the Norwegian Channel and through the Orkney­ plastic caps and sealed with tape . After sampling, the Shetland inflow . This water mass enters the Skagerrak liners were stored upright at 4° C. Sedimentation rates along the sout hern slope as the Atlantic Shelf Edge were determined using a- and y-spectromet ry. 21° Pb was Current. Water com ing from the south ern North Sea measured via its a-particles emitting granddaughter enters the Skagerrak along the southern slope and part ly 21OPO, which is assumed to be in equilibrium with 21°Pb. flows into the Kattegat. The remain der mixes w ith water The sedimentat ion rates were calculated according to the coming from the Baltic Sea, and the Atla ntic Shelf Edge (IC (Constant Init ial Concentration) method. The method Current. This water mass leaves the Skagerrak as the less used is described in mo re deta il by van Weering et al. saline Norwegian Coastal Current, w hich flows to the (1987). 137CS activity was measured using Aptec and no rth t hrou gh t he Norw egian Channel and fin ally into Canberra germanium y-ray detectors on 1 cm-thick sedi­ the Norwegian Sea . Seasona l variations in this patte rn are ment slices with 35-65 g dry weight, depending on the the result of differences in water temperatu re and wind porosity of the sediment. 21°Pb and 137CS analyses were st ress (Otto et al. 1990). The fraction of th e sediments perfo rmed on the selected cores at 1 cm depth intervals. which does not settle in the Skagerrak is transported out The counting time for the a- and y- measurements was 24 of the Skagerrak into t he Norwegian Channel by th e hours and 1 to 2 weeks, respectively. Norwegian Coastal Current as suspended load. Some of X-radiographs of the split cores were made using a th is material sett les in the Norwegian Channel and the Hewlett-Packard 43805N X-ray system Faxitron. The radi- remainder is transported into t he Norwegian Sea (Eisma 1981, van Weering 1982). NGU-BULL 430. 1996 Aivo Lepland & Rodney L.Stevens 59

ENAM 18 Percentage 25 50 751 0

4 ENA M 16 EN AM 15 ENAM 14 Percentage Percentage Percentage 25 SO 75 100 25 SO 75 100 25 50 75 100

ENAM 10 Perce ntage 25 50 751

s: Q.., o

ENAM 9 ENAM 8 Percen tage Perce ntag e 0...5 - 411m 4 · 6411m 25 50 75 1 0 25 50 75 100 f77721..' ..••.. • Fig. 3. Results of the gra in -size analyses of rLL&1 the ENAM cores. Position of the graph s rela ­ 64 - 125 11m 125 - 188 11m tive to the geograph ical location of the cores.

Results cores showed worm tubes at the sediment surface. The brownish colours of most of the top sediments of the Sediments cores indicate a well oxygenated environment.

Grain-size analysis and dry bulk density Description

The sediments in the Norwegian Channel consist mainly The grain sizes have only been determined on the of brownish, greenish and greyish silty clays. Some cores ENAM93 samples (Fig. 3). Most of these cores consist of show a few cm th ick sandy intervals (e.g. Sk91-4), while clayey silts, the percentage of clay being usually lessthan others are sandy th roughout the core (e.g.Sk91-6). Core 25%. In most of t he cores the amount of very fine to fine Sk91-2, taken high upon t he western slope of the sand is 5% or less. Cores ENAM93-10, 13 and 16 are relati­ Norweg ian Channe l, consists of med ium to coarse sand. vely coarse, and the amount of sandy material sometimes Sometimes gravel is found (core NG88-6). Shells, shell exceeds 40% (top of ENAM93-10). The finest-grained fragments and echinoid spines are common. sediments are found in the central part of the Norwegian Macroscop ically the cores are homogeneous and lack Channel. The grain-size distribution of the surface sedi­ almost completely primary sedimentary structures. X­ ments is in agreement with the data provided by Qvale & radiographs of th e cores also show very few primary sedi­ van Weering (1985). mentary st ructures. Coarse-grained intervals, quartz and Dry bulk densiti es vary from 0.35 to 1.79 qxcrrr depen ­ clay pebbles, and dark and light intervals caused by diffe­ ding on the depth in t he core and the grain-size distribu ­ rences in mineralogy can be recognised on th e x-radio­ tion. graphs. Macroscopic burrows can be recogni sed in mo st of the cores and on the X-ray images. The burrows have diameters rang ing from 0.5 mm to more th an 10 mm. In 210 Pb and 137Cs measurements and many cases on ly a few narrow (0 =0.5 mm ) or wi der (0 sedimentation rates ± 10 mm ) burrows are present, leaving t he few primary sedimentary structures and the light and dark intervals The locations of the individual stations and the sedimen ­ intact, i.e., the sediments are more or less und isturbed. In tation rates measured are given in Table 1. Cores other cores the primary structures and colour banding ENAM93-8, ENAM93-15, NG88-6, NG88-7, NG88-10, are clearly disturbed, indicating strong bioturbation NG88-15, Sk91 -13 and Sk91 -14 were not used for inter­ (cores ENAM93-8, Sk91-13 and Sk91-14). Several box pretations, because they are strongly bioturbated. Core 60 Aivo Lepland &Rodney L.Stevens GU-BULL <:30, 1996

ENAM 9 ENAM 13 01 2,oPb excess activity (Bq 'kg" ) 210Pb excess activity (Bq 'kg ) ENAM 9 ENAM 13 2 3 01 2 10" 10° 10' 10 10 10 10° ld 10 ld Ac tiv ity (Bq/k g) Ac tivit y (Bq / k g) SML SML 0 10 0 10 0 0 SML _ 196 3 2 2 - , 963 E E j-""'"' E.. E.. E 4 E 4 y"" ~ 2 ~ ~ .c s: 0'" li li Q) 6 ID 6 I 0 0 3 • I 8 8 I

4 • 10 10

ENAM 14 ENAM 16 O O ENAM 14 ENAM 16 210Pb exce ss activity (Bq 'kg ' ) 21 0pb excess activity (Bq 'kg ' ) 01 0 3 Activity (Bq / k g) Act ivity (Bq/kg) 10"' 10° 10' 102 103 10 10 10' 102 10 SML 0 10 0 10 SML 0 SML 0 SML ee 2 X.. 2 - 1963 >- ' ~ ' E E 4 E 4 E.. ~ - ~ "% 2 • x: s: Q) li I li 0 Q) 6 Q) 6 I - , 96 3 0 0 I I 3 8 I 8 I I J 10 10 4 ~

Fig. 4. Selected results of the lIOPb measureme nts. Grey in terval indicates Fig. 5. Results of"'Cs m easurem ents ofENAM 1993 cores. Grey interval indi­ thickness ofthe surface mixed layer (SML). See text for more details. cat es th ickn ess of th e surface mixed layer (SML). Arrows indicate expected depth of 1963 and 1986 activity peaks.

Sk91-2 was on ly 5 cm long and therefore to o short to be wi thin the bioturbated upper layer of the sediment and of any use. Core Sk91-4 show ed a compa ction of 53% are therefore not clearly visible in t he profile. The 1986 during sampling and storage, and is t herefo re possibly peaks in cores ENAM93-13 and 14 are wi thin the biotur­ too disturbed to allow a reliable deter mination of the bated layer, but small increases in 137Cs-act ivity are visible. sedimentation rate. Fig. 4 shows some of the results of Bot h th ese cores show a maximum in activity resulting the 2lO Pb measurements. In general , th e logarit hmic from the 1963 nuclear bomb testing.The peak at t he base excess 210 Pb activity plotted versus core depth shows a of the bioturbated layer of core ENAM93-16 might be the straight line below t he surface mixed layer (SML). Usually result of the buria l of 1986 sediment s du e to bioturbati­ the SML is very th in. In most cores th e t hickness does not on. This causes a pre-1986 137Cs-activity peak whi ch is exceed 2 cm (Fig. 4). In some cores, howeve r,(ENAM93-8 located too deep in the core. The high activity in the 3-4 and 15 ) the SML reaches a th ickness of 15 cm or more. cm interval just below the bioturbated layer may be the Some of the 2,oPb plots show anomalously high or low result of the increased Sellafield releases w hich peaked 2,o Pb values deepe r in the core (ENAM93-13, Fig. 4). These around 1975 to 1980 (Kunzendorf et al. t his volume). The are thought to be the result of local burrow ing below the sediment ation rates were determined from the 2'OPb and SML, whi ch resulted in the transport and subseque nt 137Cs measurement s. The sedimentation rates in the mixing of older or younger sediment. This explanation is Norweg ian Channe l range from 30 to 280 mm xlOO yr' . supported by t he X-radiogra phs whi ch show some major Highe st sedimentation rates were found in the northern burrows at t he dept hs conce rned, and by grain-size ana­ part of t he Norwe gian Channel (Fig. 2). lyses showing a variance in grain size at these specific From the literature it is know n that on the Fladen depths. Ground, on th e plateau northwest of the Norweg ian Fig. 5 shows 137Cs-act ivity profi les of some of th e Channel and th e upper western slope of t he Norweg ian ENAM93 cores. The 1963 nuclear bomb testing and th e Channel, there is little sedimentation, and possibly erosi­ 1986 Chemobyl P'Cs-activity peaks in core ENAM93-9 fall on (Johnson & Elkins 1979, Rise & Rokoengen 1984). On NGU-BULL 430, 1996 Aivo Lepland &Rodney L.Stevens 61

o 2 4 6 the eastern slope of the Norwegian Channel, to wards the mainland, crystalline bedrock extending from land is locally exposed, which indicates that non-depositional conditions prevail there (Holtedahl 1993, Rise & Roko­ engen 1984), but isolated, ponded Holo cene is likely to 62 be present. On the western slope of the channel t he local effect of erosion on the platform is not iced in the presen­ ce and amount of older, reworked foraminifera in Nor­ wegian Channel sediments (Qvale & van Weering 1985).

Acoustical data

In the period 1973-1976, 3.5 kHz echo-sounding lines were recorded across the Norwegian Channel between 0N 60 58° and 63°N. Lines located north of 60 are discussed in van Weering (1983). In 1993, some add itiona l lines were recorded using a Datasonics Chirp echosoun der. Together t hese lines cover a length of more than 5500 km (Fig. 6). North of 600N van Weering (1983) identified four sedi­ mentary units reflecting depositional events during and after the last glaciation. These units can be traced in the area south of 600N. The upper unit (unit 1) represents Holocene fine-grained sediments wh ich overlie Weichselian glacial marine deposits (unit 2)(van Weering 58 1983, Holtedahl 1993). The Holocene and glacial marine sediments are not equally thick throughout the area. The average thi ckness of the combined units in t he central part of t he Norwegian Channel is abo ut 10-20 m. Datings of a core from the central part of the Norwegian Channel show that here the base of the Weichselian glacial marine Fig, 6. Location map of the acoustica l reflection profiles used in thi s study. unit lies at about 23 m, and the base of the Holocene at Numbers 7A to 8 indicote the position of the profiles show n in Figs 7A to 8, respectively. 3.75 m depth (core Troll 3.1 , Lehman et al. 1991, Lehman & Keigwin 1992). In the southern part of the Norwegian

sw s 5 km 004 N A NE B "I" ,.' k m 004 "I 50m .! 50 m L ate W eichsel ia nl .! Holoce ne co ver

c

Fig. 7. 3.5 kHz penetrating echosounder profiles from the Norwegian Channel. Dashed lines in subsurface are tim e markers. A) Profile showing th e pinching out of the Holocene sediments on the western slope of the Norwegian Channel. Water depth 320-390 m, vertical exaggeration 20x. B) Profile showing thin Holo cene sedimentary cover ona topographic high in the centra l part of the Norwegian Channel. Water depth 400-440 m, vertical exaggeration SOx. C) Profile showing a smallloeal basin on the eastern slope ofthe Norwegian Chann el. Water depth 340-420 m, vertical exaggeration 20x. 62 Aivo Lepland &Rodney L.Stevens NGU-BULL430, 1996

2 m s .." .. N I E W 50 m .,1 <> ~I[ 0-<= B " I" C

A B c

Fig. 8. Penetrating echo sounder profi le showing the truncation ofint ernal reflectors a t the sea floor, for details see blow ups A to C. Arro ws indicate the trun ­ cation ofinternal reflectors by the sea-floor reflector. Water depth 240-260 m, vertical exaggeration 30x.

o 2 4 6 Channel these layers can be thicker, but th e boundary between the Holocene sediments and th e Weichselian glacial marine sediments beneath (van Weering 1983, unit 2) can be very unclear. The thickness of the Holocene ',.:.-1 cover can therefore not always be determined accurately. 62 Units 1 and 2 pinch out towards the margins of the Norweg ian Channel and to wards t he south and the nort h (Fig. 7A). On topographic highs the Weichselian and Holocene sedimentary cover is sometimes extre mely th in (Fig. 7B). Local depressions of the sea bed act as sedim en­ tary basins in which increased sedimentation occurs. Especially along the eastern flan k of the Norwegian Channel the pro nou nced topog raphy results in th e pre­ sence of small local basins (Fig. 7C). In som e cases the internal reflectors of earlier deposited sediments appear to be truncated at the seafloor (Fig. 8). 60 Accumulation rates

North Sea Based on t he acoustica l data, the maximum surface area of th at part of the Norwegian Channel in wh ich recent sedimentation occurs is calculated to be 33,900 km' (Fig. 9). Using three intervals of sedimentati on rates (0-100, 100-200 and >200 mm x100 yr') and calculat ing average sedimentation rates and average dry bulk densitie s fo r each ofthese intervals, t he maximum tota l yearly dry bulk 584------accumulat ion rate in this part of the Norweg ian Channel o 0-10cm'100 y(l is calculated to be 28x106 tons . D 10-20 cm .100 y(l • >20cm .100 y ( l Discussion

Alt hough the X-radiographs clearly show th e effect of Fig. 9. Geographical distribution of recent sedimentation areas in the bioturbation, th e sedimentation rates calculated from Norwegian Channel. NGU-BUL L430, 1996 Aivo Lepland & Rodney L.Stevens 63

Tab le 1. List of stations and "'Pb sed imentat ion rates (n.d.=not determi- explanation is supported by the grain-size distribution in ned). two piston cores taken about 30 miles NNW (station Sedime ntatio n ENAM93-15, Table 1) and 45 miles SSW (ENAM93-11, Sta tion Position rate Table 1) of core Troll 3.1 described by Lehman et al. No. North East (mm/100 yr) (1991) and Lehman & Keigwin (1992). In these two cores a Sk91-2 61"20.45' 2°00.70' n.d. coarsening upwards starts at -1 m and -2 m core depth, Sk91-3 61"30.79' 2° 33.42' 190 respectively. In both ENAM cores the grain size increases Sk91-4 61"39.20' 3°24.90' n.d. from clayey silt to coarse silt - very fine sandy silt, Echo­ Sk91-6 62007.00' 1° 19.78' 190 Sk91-7 62"09.83' 2° 18.83' 220 sound ing lines located very near the three cores mentio­ Sk91-8 62 ' 12.28' 3° 17.75' 120 ned, do suggest that on the locati ons where the two Sk91-9 62 '12 .81' 3°38.05' 60 ENAM93 cores and core Troll 3.1 were taken , similar sedi­ Sk91-10 60°43.50' 4° 29.70' 30 Sk91-11 60°42.7 1' 4° 10.28' 280 mentary proce sses took place during the Late Sk91-12 60°43.49' 3° 20.89' 60 Weichselian and Holocene.The only difference is found in Sk91-13 60°42.62' 3° 10.24' n.d. the total thickness of the sedimentary unit. In the south Sk91-14 60°43.30' 3° 13.08' n.d. Sk91-15 59°30.60' 4°46.50' 120 (near ENAM93-11), the unit is about 1.5 times as thick as Sk91-16 59"29.99' 4° 27.30' 90 on the location of core Troll 3.1. In the north, near station NG88-6 58°10.47' 4° 37.61' n.d. ENAM93-15, the thickness is just slightly less than near NG88-7 58°12.43' 4°40.25' n.d. NG88-8 58°15.73' 4° 52.12' 100 core Troll 3.1. The mechanism behind th is change in NG88-9 58"24.18' 5°07.83' 110 depositional regime and possible related change in NG88-10 58"28.61' 5° 18.92' n.d. hydrological conditions is not yet known. Further study NG88-11 58"36.68' 5° 24.71' 120 NG88-12 59'11.92' 4° 51.39' 40 of the piston cores will most probably give the answer to NG88-13 59°13.81' 4°26.94' 60 th is question. NG88-14 59°15.47' 4°08.85' 40 Van Weering (1983) concluded that in the northern NG88-15 59°12.70' 3°42.88' n.d. NG88-16 59°13.07' 3° 30.46' 90 part of the Norwegian Channel the deposition of the NG88-17 59°59.45' 3° 17.80' 140 Holocene deposits (unit 1) ceased around 8000 BP. The NG88-19 60001.72' 3°55.14' 160 results of the 210 Pb measurements presented here clearly ENAM 8 59"30.10' 3° 41.25 ' n.d. ENAM 9 59"30.03' 4° 05.29' 40 show that this is not the case, as was already suggested ENAM 10 60"20.60' 4° 39.32' 60 by Nagy & Ofstad (1980). The absence of present-day ENAM 11 60°19.98' 3°22.06' n.d. sedimentation along the eastern flank of the Norwegian ENAM 13 61°19.94' 3° 52.17' 70 ENAM 14 6 1°19.77' 3° 35.58' 100 Channel, as concluded on the basis of the echo-sounding ENAM 15 61°19.25' 3° 15.87' n.d. data (Fig. 8), is supported by the work of Nagy & Ofstad ENAM 16 61°19.96' 2°3 4.64' 110 (1 980) who stud ied foram inifera along two transects ENAM 18 62003.69' 2° 55.58' 190 across the Norwegian Channel. Several authors have reported erosion and redistributi­ on of sediments on the shallow seafloor of the North Sea. Erosion of the Flemish Banks (sout hern North Sea) was downcore 21°Pb profiles are considered to reflect the real reported by Eisma&Kalf (1987).The tidal current near the sedimentation rates. This assumption is based on the Dogger Bank is not strong enough to transport sea-floor work of Nittrouer et al. (1984), who concluded that sedi­ sediments. Sea-floor erosion and sediment transport in mentation rates on the Washington shelf can be calcula­ this area, however, do occur during storms (von Haugwitz ted from the 21° Pb profile below the surface mixed layer. & Wong 1988), Sea-floor photographs of the Norwegian Furthermore, the maxima in the 137Cs-activity profiles sector of the North Sea indicate local present-day sea-flo ­ coincide well with the expected depths of the 1963 or erosion (Rise & Rokoengen 1984, NIOZ unpublished nuclear bomb test and 1986 Chernobyl accident peaks, data). Foramin iferal studies also point to local erosion thus supporting the sedimentation rates determined by and reworking of sediments (Qvale & van Weering 1985). the 21°Pb method. Sediments removed from the sea floor of the southern The thin Holocene sedimentary cover in the central North Sea are transported northwards by the residual part of the northern Norwegian Channel, in the order of 4 current (Eisma & Kalf 1987). Erlenkeuser & Pederstad m or less,and the even thinner layer near the flank s of the (1984) found fine -grained sediment from a southern ori­ Norweg ian Channel do not correspond with the extrapo­ gin (southern North Sea and the Baltic Sea) in the lated recent sedimentati on rates measured by the 21°Pb Skagerrak. Hass(1993) concluded that the sedimentation method, which are in the order of 100-200 mmxl00 yr '. pattern in th e Skagerrak is influenced by short-term cli­ The relativ ely th in Holocene sedimentary cover in relati­ matic change s. He show ed t hat the average sedimentati­ on to the high recent sedimentation rates in t he northern on rates in a core in the Skagerrak clearly increased part of the area cannot be explained by a str onger com­ during the stormy ' Little Ice Age'(1550-1890 AD.) com­ paction of the sediments. The contrast between the pre­ pared to the four centuries before the Little Ice Age and sent-day sedimentation rates and the thin Holocene sedi­ the present-day situation. An increased frequency of mentary cover can, however, be explained by a change in winds influenced the bottom currents and therefore ero- sedimentary regime sometime during the Holocene. This 64 Aivo Lepland & Rodney L. Stevens GU-BULL430. 1996

sion and t ransport of sediments. The sediment s in the cript. Birthe Bak checked th e English text. This research is supported by Norwegian Channel are tran sported and supplied by th e the Netherlands Organ isation for the Adva ncement of Science (NWO) under gran t 753-718-229 to Tj.e.E.v.w. This is NIOZ contribution no. Jutland Current, the Baltic Current , and also by the 3093. Norwegian rivers and fjord s (Bj6rklund et al. 1985); th us sediments eroded from the North Sea sea floor and sedi­ ments t ransported out of the Balti c Sea may be deposited References in the Norwegian Channel. In the Norwegian Channel, w here current veloc iti es are lower and wave acti vity less Bjiirklund, K.R., Bjcrn stad, H., Dale, B., Erlenkeuser, H., Henning smoen, pronounced, the sediments settle out of suspension and K.E., Hoeg, H.I., Jonsen, K., Manum, S.B., Mi kkelsen, N., Nagy, J., in the deeper parts of the Norwegian Channel the hydro­ Pede rstad, K., Ovale , G., Rosenqvist, I.T., Salbu , B., Schoenha rting, G., Stabell, B., Thiede, J., Thro nd sen, I., Wassmann, P. & Werner, F. 1985: logical conditions favour permanent deposition of the Evolu tion of the Upper Quaterna ry depositional envi ronment in the sedimen ts. This is clearly seen in Fig. 7A, whi ch shows Skagerrak: A synthe sis. Nor. Geol. Tidsskr. 65, 139·149. that depo sition occur s on ly in the lower parts of the Den neqard, B., Jensen, A., Kuypers, A. & van Weering, T.e.E. 1992: flanks. Another examp le is given in Fig. 7B, which shows Quaternary sediment accumulation and recent sedimentary pro­ cesses in the Skagerrak and northern Kattega t, an overview. Report that on local topographic highs in the deep central part 7, University ofGoteborq. Department of Marine Geology, 164 pp . of the st udy area the sedimen tary cover is much th inner Eisma, D. 1981: Supp ly and dep osit ion of suspend ed matter in the North than in th e surrounding deeper parts. Local dep ressions Sea. Spec. Publ. Int. Ass. Sediment. S, 415-428. Eisma, D. & Kalf, J. 1987: Dispersal, conc entration and deposi tion of sus­ higher up the slopes provide enough shelte r for sedi­ pended matter in the North Sea.Jour. Geol. Soc. Lond. 144, 161-178. ment s to settle permanent ly, as is clearly demonstrated Erlenkeu ser, H. 1985: Stable isot op es in benthic fo raminifers of in Fig. 7CThe trunca tion of reflectors within older depo­ Skagerrak core GIK 15530-4: High resolut ion record of the Younger sits shown in Fig. 8 indicates th at previously deposited Dryas and the Holocene. Nor.Geol. Tidsskr. 65, 50-57. Erlen keuser, H. & Pederstad, K. 1984: Recent sedi ment accumulation in sedim ent s are eroded. Skagerrak as dep icted by "'Pb-dating. Nor. Geol. Tidsskr. 64, 135-152. Hass, H.e. 1993: Depositional processes under changing climate: Upper Subatlantic granulom etr ic records from the Skagerrak (NE·Nort h Conclusions Sea). Mar. Geol. 111,361-378. Henningsmoen, K.E. & Hoeg, H.J. 1985: Pollen analyses from the the Skagerrak core GIK 15530-4. Nor. Geol. Tidsskr. 65, 41-47. Downcore 210 Pb activity measurements allow the determi­ Holte dahl, H. 1993: Marine geology of the Norweg ian conti nental mar­ nation of recent sedim ent ati on and mass accumulation gin. Nor. geol. unders. Speciol Publ. 6, 1-150. rates in th e Norw egian Channel. The recent sediment ati­ John son, T.e. & Elkins, S.R. 1979: Holocene deposits of the northern North Sea: evidence for dynam ic control of their mineral and chemi­ on rates in t he Norwegian Channel range from 30 to 280 cal composition . Geol. Mijnbouw 58,353-366. mm x100 yr' . Highest sedimentation rates are found in Jorgensen, B.B., Bang, M. & Blackburn , T.H. 1990: Anaerobic mineralizati­ the northern part of th e Norw egian Channel. Sedim ents on in marine sediments from the Baltic Sea-North Sea tra nsit ion . Mar. Ecol. Prog.Ser. 59, 39-54. ente r th e Norwegian Channel fro m the south t hrough the Kunzendorf, H., Longva , O. & Paetzel, M. 1996: Recent sedimentati on Skagerrak and from the west from t he North Sea plateau, rates across the Norw. most probably as suspended load. In th e Norwegia n vol um e. Channel, lower hydrodynam ic act ivity allows the sedi­ Lehman ,S.J., Jone s, G.A., I 0stmo, S· R. 1991: InillOllVI I VI I C IIIIU.>\..UIIUIQ. I , .... c -,I IC Cl l~ l lC O l ment s to settle out of suspension. Recent deposit ion of du ring the last deglaciatio n. Nature 349, 513-516. sediments occurs in the deeper central part and in small Lehman, SJ. & Keigwin, L.D. 1992: Sudden Change s in North Atlantic cir­ protected basins higher up on th e eastern flank of the culation du ring the last deglac iat ion. Nature 356, 7S7·762. channel. In the centra l northern Norweg ian Channel the Long, D., Bent, A., Harland, R., Gregory, D.M., Graham , DK & Morton, A.e., 1986: Late Quaternary paleontology, sedimentol ogy and geo­ t hickness of the Holoce ne sedim entary cover is generally chemistry of a vibrocore from the Witch Ground Basin, central - 4 m, and become s th inner to wards th e flan ks of the North Sea. Mar. Geol. 73, 109-123. basin. In the southern part th e Holocene deposits are Long , D., Laban, C, Strif, H., Cameron , TD .J.& Schuttenhelrn, R.T.E. 1988: t hicker. The contra st between th e thin Holocene cover The sedimentary record of climatic variatio n in the southern North Sea. Phil. Trans.R. Soc. Lond. 8318, 523-537. and t he relatively high present -day sedimentation rates is McCave, I.N. 1973: Mud in the North Sea. In Goldberg. E.D. (ed.): North explained by a change in the hydr olog ical and depositi o­ SeaScience. MIT Press, Cambr idge , Mass., 75-100. nal regim e in th e Norwegian Channel som etime during Moodley, L. & van Weering , T.e.E. 1993: Foram in iferal record of the Holocene development of th e marine envi ronment in the southern the Holocene. In the Norwegian Channel 28x10· tons of North Sea. Neth.J. SeaRes. 3 I, 43-52. dry bulk sediment is deposited annually. Nagy, J. & Ofstad , K. 1980: Quaternary forami nifera and sedi ments in the Norw egian Channel. Boreas 9, 39-52. Nittrouer, e.A., DeMaster, DJ ., McKee, BA, Cutshall, N.H. & Larsen, I.L. 1984: The effect of sediment mixing on Pb·21O accumulation rates Acknowledgements for the Washi ngto n Shelf. Mar. Geol. 54, 201-221 . Otto , L., Zimmerman, J.T.F., Furnes, G.K., Mo rk, M., Saetre, R. & Becker, G., We thank the captai n and crew of the RV. 'Pelagia' for th eir assistance 1990: Review of the physical oceanog raphy of the North Sea. Neth. and for a pleasant stay on board du ring the cruises. Jack Schilling , J. SeaRes. 26, 161-238. Cerillo Willems, Ton Kuypers, Troels Laier and Yvo Kok are thanked for Pedersen, T., Pett erson, S.E . & Husebye, E.S. 1991: Skagerrak evolu tion th eir assistance duri ng sampling. Wim Boer assisted wi th the '''Cs mea­ derived from tect onic subside nce. Tecronophysics 189, 149-163. surements. Grain-size analysis was carried ou t by Marjan Reith Peder stad, K., Roaldset, E. & Ronningland, T.M. 1993: Sedimentation and (Depart ment of Earth Scien ces, University of Utrecht). The critical revie­ environm ental condi tions in the innner Skagerrak-ou ter Oslofjord. wi ng by Helm ar Kunzendorf and Edward King improved the manu s- Mar.Geol. 111,245-268. NGU-BULL 430, 1996 Aivo Lepland & Rodney L. Stevens 65

Qvale , G. & van Weering, T.CE. 1985: Relationship of surface sediments mulation in the Skagerrak, northeastern North Sea. Neth . J. Sea Res. and benthic formaniniferal distribution patterns in the Norwegian 27,177-189. Channel (northern North Sea).Mar. Micropal. 9, 469-488. Van Weering, T.CE., Berger, GW. & Okkels, E. 1993: Sediment transport, Rise, L. & Rokoengen, K. 1984: Surficial sediments in the Norwegian sec­ resuspension and accumulation rates in the northeastern tor of the North Sea between 60°30' and 62° N. Mar. Geol. 58, 287­ Skagerrak. Mar. Geol. 111,269-285. 317. Van Weering, T.CE. & Qvale , G. 1983: Recent sed iments and foraminife­ Van Weering, T.CE. 1975: Late Quaternary history of the Skagerrak; an ral di stribution in the Skagerrak, northeastern North Sea. Mar. Geol. interpretation of acoustical profiles. Geol. Mijnbouw 54, 130-145. 52,75-99. Van Weering, T.CE. 1981: Recent sediments and sediment transport in Veenstra, H.J. 1965: Geology of the Doggersbank Area, North Sea. Mar. the northern North Sea; surface sediments of the Skagerrak. Spec. Geol. 3, 245- 262. Publ.lnt. Ass. Sediment. 5, 335- 359. Von Haugwitz, W. & Wong, H.K. 1988: The Dogger Bank: Seismic strati­ Van Weering, T.CE. 1982: Recent sed iment s and sediment transport in graphy and Holocene sedimentation. Mitt. Geoi.Palaont. Inst. Univ . the North Sea; pistoncores from the Skagerrak. Proceedings Kon. Hamburg 65,381-407. Ned. Ac. Wetensch. 885, 155-201. Zagwijn, W.H. & Veenstra, H.J. 1966: A pollen-analytical study of cores Van Weering , T.CE. 1983: Acoustical reflection profiles, sediments and from Outer Silver Pit, North Sea.Mar. Geol. 4, 539-551. late Quaternary history of the Norwegian Channel north of Bergen. Zuo, Z., Eisma, D. & Berger, GW. 1989: Recent sediment deposition rate s Geol. Mijnbouw62, 219-328 . in the Oyster Ground, North Sea.Neth. J. Sea Res. 23, 263-269. Van Weering, T.CE., Berger, G.W. & Kalf, J. 1987: Recent sediment accu-

Manuscript received Apri/1 995; revised version accepted November /995.